Vibration signals, such as sound signals, are transmitted between different points under many circumstances. Many of these require transcoding and/or amplification. For example, orchestras and musical groups play in public, and their sounds must be amplified for a group of listeners; telephones and radios transmit voices and music over long distances, the first via wires, the second via radio waves; hearing aids amplify sounds collected from the user's environment and deliver them to the eardrum or to said user's ear bone structures; television takes images collected using a video camera, transforms them into electronic signals and then recreates them on viewers' screens after decoding.
In all instances, the signals are collected at the transmitting point, transformed into electronic signals, generally amplified, and then reconstituted at the reception point.
The transducers that transform the mechanical vibrations into electronic signals (as is the case for microphones), those that transform the electronic signals into mechanical vibrations (as is the case for speakers), and the devices and components that connect these transducers in a complete system are made from a wide range of materials and active and passive circuits.
The lack of homogeneity that results from these multiple elements has a direct influence on the transmission of signals between the “input” transducer and the “output” transducer, such that the signals are not transferred in a linear manner, making their transfer efficiency variable depending on the frequency used.
For a transducer of any kind, the level of reproduction of the signal based on its frequency must be established, yielding a curve called the “characteristic curve.” A device that integrates such a transducer must include methods that allow this curve to be monitored in order to correct, to the extent possible, problems that may occur in signal reproduction.
In addition, during the signal transfer there is a phenomenon that occurs wherein a fraction of the signal transmitted and received by the output transducer returns to the input transducer and is added to the main signal. This phenomenon generates instability in the system and tends to cause signal fluctuations, especially at higher yield frequencies; the more that energy increases, the greater the level of feedback.
The most well-known manifestation of this phenomenon is called the “Larsen effect” or “Larsen.” It occurs when input signals, such as voice signals, picked up by, for example, a microphone, are amplified, transmitted to a speaker, and then returned to the microphone which captures them along with the new signals. The new and returned signals are then reamplified, which, due to the non-linear nature of the elements making up the transfer chain, results in a fluctuation of the whole signal, which in turn results in a very loud screech that is characteristic of the Larsen effect.
The microphone thus “hears” not only the voice, but also the speaker, and this effect increases with greater microphone sensitivity, greater speaker volume and shorter distance between the microphone and the speaker.
This phenomenon may be created at will and observed by bringing the microphone of a telephone handset close to a speaker plugged into an amplifier.
There are several known methods for addressing the problems created by this feedback:                limiting the microphone sensitivity, the theory being that by reducing the input signal, the sounds coming from the speaker will not be picked up;        limiting the speaker power, the theory being that by reducing the output level, the sounds from the speaker will not reach the microphone; and        increasing the distance between the microphone and the speaker, or facing them in specific directions, the theory being that reducing the physical proximity between the microphone (input transducer) and the speaker (output transducer) may prevent the sounds from the speaker from being picked up by the microphone.        
All of these methods help reduce feedback, but the limitations that they impose often limit the system capabilities and reduce the expected quality. Items such as portable wireless telephones (cell phones) or, to an even greater extent, hearing aids must be contained in the most compact structure possible, which is completely incompatible with the concept of keeping a large distance between the microphone and the speaker (or earpiece in this case). As a result, in such devices, sound levels must automatically be kept low, which is not satisfactory since it limits the possible design options for the device.
Another method for addressing the Larsen effect consists of filtering the signals at one or multiple points in the transfer chain, in order to “trap” the fluctuations. This method is not very effective and it contributes significantly to the non-linear nature of the entire device since the filters themselves are some of the most non-linear devices made. Another disadvantage of this method is that it results in a significant distortion of the output signals, which changes the transmission and seriously affects the qualitative characteristics of the input signals, such as the elimination of treble, muffling, etc.
In the field of telecommunications, feedback has such negative consequences, such as muffling of sound, that duplex links are simply prohibited for critical applications, such as communication of military information. For these applications, duplex links are replaced by “alternative bilateral” links in which only one of two speakers is permitted to talk at a time, or alternatively, the other can listen, but must wait to speak until there is a pause or the speaker will be abruptly interrupted. This is extremely inconvenient and even unusable in certain situations.